You're sure that's not 12.75 pre and 11.5 post? It's got to be accurate.
Now take your pre-boil gravity, and look just at the gravity units (GU) to the right of the decimal. So for a 1.065 beer, the GU is 65.
Okay, a bit more mathematics...
If you intended to have 13 pre/11 post with a post-boil OG of 1.065, then you would have to have 1.055 pre-boil because 55 * 13 / 11 = 65. You use the ratio of volumes to get the gravity you want with the 2-gallon boiloff that you have.
But if your actual volumes are something like 12.75 and 11.5, then look what happens for the same 1.055 pre-boil:
55 * 12.75 / 11.5 = 61 or a post-boil OG of 1.061.
Oh no, my gravity is a little low again.
You need to know all your volume measurements exactly if you want to nail your post-boil gravity.